Pulsed lasers, such as KrF excimer lasers, ArF excimer lasers and molecular fluorine (F2) lasers, are often used in conjunction with a lithography tool to selectively expose a photoresist in a semiconductor wafer fabrication process. In these processes, the mask and optics in the lithography tool are typically optimized for a particular laser wavelength. More specifically, the tool is typically optimized for a particular center wavelength. For a variety of reasons, the center wavelength of the light exiting the laser may drift over time and, thus, a feedback network may be employed to detect the center wavelength exiting the laser and modify one or more laser parameters to correct the wavelength as necessary.
Recently, optical proximity correction (OPC) has been employed to more exactly obtain a desired exposure pattern on the resist by selectively changing the sizes and shapes of corresponding patterns on the mask. More specifically, OPC perturbs mask aperture shapes to systematically compensate for nonlinear feature distortions arising from optical diffraction and resist process effects. Common types of OPC may include: (1) the introduction of serifs, hammerheads and tomahawks in the mask pattern to reduce corner rounding and line-end shortening in the resist pattern; (2) the use of notches to control linewidth accuracy; and (3) the use of sub-resolution assist features (SRAFs, or scattering bars) for narrow gate geometries.
Heretofore, efforts have been drawn largely to reducing the spectral bandwidth of the light exiting the laser source to ever smaller dimensions. In short, large bandwidths have been troublesome because, as indicated above, lithography tool optics are generally designed for a specific center wavelength and chromatic doublets are generally unavailable at the wavelengths of interest, e.g., 248 for KrF sources and 193 nm for ArF sources. More recently, and more particularly with the recent use of OPC, the design rules used to generate complex masks require bandwidth stability rather than mere bandwidth reduction. As used herein, the term “bandwidth stability” means controlling bandwidth within a spectral range having both a maximum acceptable bandwidth value and a minimum acceptable bandwidth value.
In one type of arrangement used to measure the wavelength and spectral bandwidth of a laser, a portion of the emitted light is made incident upon an etalon. The etalon creates an interference fringe pattern having concentric bands of dark and light levels due to destructive and constructive interference by the laser light. The fringe pattern may then be optically detected by a sensitive photodetector array (PDA). In the fringe pattern, the concentric bands surround a center bright portion. The position of the bright center portion of the interference pattern can be used to determine wavelength to a relatively coarse degree, such as to within 5 picometers (pm). The diameter of a light band is then used to determine the wavelength of the laser output to a fine degree, such as to within 0.01-0.03 pm. Also, the width of a light band may be used to determine the spectral bandwidth of the laser output.
In one method commonly used to characterize spectral bandwidth, the bandwidth at 50% peak intensity is measured. This definition of bandwidth is referred to as full width, half maximum (FWHM). Another definition of bandwidth, sometimes referred to as spectral purity, involves measuring a spectrum width where a selected percentage of the entire spectral energy is concentrated. For example, a common indicator is E95 which corresponds to the spectrum width where 95% of the entire spectral energy is concentrated.
Various methods can be used for wavelength tuning of lasers. Typically, the tuning takes place in a device referred to as a line narrowing package or line narrowing module. A typical technique used for line narrowing and tuning of excimer lasers is to provide a window at the back of the discharge cavity through which a portion of the laser beam passes into the line narrowing package. There, the portion of the beam is expanded in a beam expander and directed to an echelle grating that is aligned at a Littrow angle relative to the expanded beam. The grating reflects a narrow portion of the laser's untuned broader spectrum back into the discharge chamber where it is then amplified. With this arrangement, the laser can be tuned to a target center wavelength by changing the angle at which the beam illuminates the grating. This may be done by adjusting the position of the grating or providing a moveable mirror in the beam path.
Modern gas discharge lasers may be operable at a relatively high repetition rate, e.g., four to six kilohertz, and modem advancements are pushing this number higher. In a typically lithography procedure, the lithography tool may demand a burst of pulses, e.g., for exposing a particular mask position. As used herein, the term “burst of pulses” and its derivatives means a continuous train of pulses demanded by the lithography tool at the operable repetition rate of a discharge laser. A typical burst of pulses may include, but is not limited to, about 100-300 pulses. The termination of a burst of pulses may be achieved by terminating electrode discharge in the laser, the use of an appropriate shutter, or other suitable techniques known in the pertinent art. Thus, an active control system may alter a pulse characteristic within a burst of pulses by measuring a characteristic of a first pulse within the burst and using the measurement to alter a characteristic of at least one other pulse in the burst of pulses.
Typical industry specifications may require a laser output having an average center wavelength or average bandwidth (FWHM and/or E95) averaged over as a series of pulses referred to as a “pulse window”. A typical pulse window may be, for example, 30 pulses, and, in general, may be shorter than a “burst of pulses”. Thus, an active control system may alter a pulse characteristic within a pulse window by measuring a characteristic of a first pulse within the window and may be the measurement to alter a characteristic of at least one other pulse in the pulse window. Another level of active control is referred to as “pulse to pulse” control. In this technique, an active control system may measure a characteristic of a pulse and use the measurement to alter a characteristic of the very next pulse exiting the laser after the measured pulse.
With the above considerations in mind, Applicants disclose systems and methods for actively controlling the bandwidth of a laser during a burst of pulses.